1. Field of the Invention
The present invention relates to spot-welding methods and spot-welding devices for spot-welding a workpiece having a stacked-plate structure constituted of stacked plate materials having different thicknesses.
2. Description of the Related Art
Generally, when joining together stacked plate materials, such as steel plates, a spot-welding technique is widely used. Specifically, this spot-welding technique involves using a pair of welding electrodes to clamp and apply pressure to the stacked plates, applying high current between the welding electrodes for a predetermined time, and then increasing the temperature at a joint section to substantially a melting temperature so as to form a nugget.
In spot-welding, the diameter of the nugget gradually increases with increasing current when the pressure applied by the two welding electrodes and the electricity application time are fixed. However, the calorific value becomes excessive when the current value is too high, causing expulsion and surface flash, in which molten metal spatters, to occur between the plate materials. Specifically, such expulsion and surface flash is an explosion phenomenon of the molten metal caused by overheating of the joint section and causes holes and cracks to form in the nugget. This results in discontinuity in the shape of the nugget and the metallographic structure, causing a reduced thickness of the joint section as well as a significant reduction in the strength thereof. In contrast, if the current is too low, sufficient joint strength cannot be obtained since the nugget becomes small in size. If the applied pressure is low, the contact area between the plate materials correspondingly decreases, leading to the occurrence of expulsion and surface flash caused by overheating due to an increased current density. On the other hand, an excessively applied pressure leads to an increase in the contact area at the joint section, resulting in a reduced current density and a reduced calorific value. As a result, the nugget is reduced in size and the weld strength is lowered.
Referring to FIG. 18A, when spot-welding a workpiece 100 constituted of three stacked plates, which are a thin plate 101 having low rigidity, and a first thick plate 102 and a second thick plate 103 that are thicker and more rigid than the thin plate 101, a movable electrode 121 and a fixed electrode 122 are used to clamp the workpiece 100 therebetween in a state where the thin plate 101 and the first thick plate 102 as well as the first thick plate 102 and the second thick plate 103 are tightly attached to each other with no gap therebetween. Then, when a power source 123 applies electricity to the workpiece 100 via the movable electrode 121 and the fixed electrode 122, clamping the workpiece 100 therebetween, the current density in an electric path between the movable electrode 121 and the fixed electrode 122 becomes substantially uniform so that a good nugget extending from the thin plate 101 to the second thick plate 103 is formed, thereby achieving required weld strength.
In actuality, however, when the workpiece 100 is clamped and pressed between the movable electrode 121 and the fixed electrode 122, the thin plate 101 having low rigidity and the first thick plate 102 bend upward, causing gaps to form between the thin plate 101 and the first thick plate 102 as well as between the first thick plate 102 and the second thick plate 103. In this case, the contact area between the movable electrode 121 and the thin plate 101 is increased due to the bending of the thin plate 101, whereas the contact area of the joint section between the thin plate 101 and the first thick plate 102 as well as the contact area of the joint section between the first thick plate 102 and the second thick plate 103 are decreased due to the gaps.
Therefore, the current density at a joint section of the fixed electrode 122 at the side of the second thick plate 103 becomes higher than that at a joint section of the movable electrode 121 at the side of the thin plate 101. This results in a larger local calorific value between the first thick plate 102 and the second thick plate 103 than between the thin plate 101 and the first thick plate 102.
As a result, a nugget 105 is first formed at the joint section between the first thick plate 102 and the second thick plate 103, as shown in FIG. 18A. Then, the nugget 105 gradually increases in size so that the thin plate 101 and the first thick plate 102 are ultimately welded to each other, as shown in FIG. 18B. However, because the amount of weld penetration between the thin plate 101 and the first thick plate 102 is small, the weld strength is unstable. Thus, the thin plate 101 may become delaminated, and the weld quality may vary from place to place. This problem is prominent especially with increasing thickness of the first thick plate 102 and the second thick plate 103 since this makes it difficult for the nugget 105 to reach the joint section between the first thick plate 102 and the thin plate 101.
Another factor that makes the weld strength unstable due to a small amount of weld penetration between the thin plate 101 and the first thick plate 102 is a small thickness of the thin plate 101. Specifically, such a thin plate 101 with a small thickness surrenders its heat to the movable electrode 121 by being in contact therewith and therefore does not increase in temperature, making it difficult to form the nugget 105.
Japanese Unexamined Patent Application Publication No. 2003-251468 discloses an example of a spot-welding method as a countermeasure against this problem. Specifically, as shown in FIG. 19, when spot-welding the workpiece 100 constituted of stacked plates, i.e., the thin plate 101, the first thick plate 102, and the second thick plate 103, the tip diameter of the movable electrode 121 that comes into contact with the thin plate 101 is made smaller than the tip diameter of the fixed electrode 122 that comes into contact with the second thick plate 103 so that the contact area between the thin plate 101 and the movable electrode 121 is smaller than the contact area between the second thick plate 103 and the fixed electrode 122. Thus, the current density in the electric path between the movable electrode 121 and the fixed electrode 122 gradually decreases from the movable electrode 121 towards the fixed electrode 122. As a result, the calorific value between the thin plate 101 and the first thick plate 102 becomes larger so that a good nugget is formed, whereby the weld strength between the thin plate 101 and the first thick plate 102 is increased.
Japanese Unexamined Patent Application Publication No. 2003-251469 discloses another spot-welding method. Specifically, as shown in FIG. 20, when spot-welding the workpiece 100 constituted of three stacked plates, i.e., the thin plate 101, the first thick plate 102, and the second thick plate 103, a pressure FU from a movable electrode 135 located at the thin plate 101 side is set to be smaller than a pressure FL from a fixed electrode 134 located at the second thick plate 103 side so that the contact resistance between the thin plate 101 and the first thick plate 102 increases, whereas the contact resistance between the first thick plate 102 and the second thick plate 103 decreases. Thus, when electricity is applied between the movable electrode 135 and the fixed electrode 134, the calorific value at the joint section between the thin plate 101 and the first thick plate 102 is increased, thereby increasing the weld strength between the thin plate 101 and the first thick plate 102.
FIG. 21 illustrates the configuration of a spot-welding device used for implementing this method. Specifically, a spot-welding device 130 is attached to a wrist portion 126 of a welding robot 125. The welding robot 125 spot-welds the workpiece 100 by moving the spot-welding device 130 to each spot-welding position of the workpiece 100 supported by a clamper 128.
The spot-welding device 130 includes a base 132 that is supported in a vertically movable manner by a linear guide 131 fixed to a support bracket 127 attached to the wrist portion 126. The base 132 is provided with a C-shaped yoke 133 that extends downward therefrom. A lower end of the C-shaped yoke 133 is provided with the fixed electrode 134.
A pressure actuator 136, such as a servomotor, is attached to an upper-end of the base 132. The movable electrode 135 is attached to a lower end of a rod 137 that is moved in the vertical direction by the pressure actuator 136, such that the movable electrode 135 faces the fixed electrode 134. A servomotor 138 is attached to an upper end of the support bracket 127. By actuating the servomotor 138, the base 132 is moved in the vertical direction via a ball-screw mechanism.
Based on teaching data preliminarily stored in a controller (not shown), the pressure FU from the movable electrode 135 located at the thin plate 101 side is set to be smaller than the pressure FL from the fixed electrode 134 (FU<FL).
In order to set the pressure FU from the movable electrode 135 to be smaller than the pressure FL from the fixed electrode 134 (FU<FL) in this manner, the controller first uses the servomotor 138 to move the base 132 upward so as to bring the fixed electrode 134 into contact with the lower surface of the workpiece 100, and also uses the pressure actuator 136 to move the movable electrode 135 downward so as to bring the movable electrode 135 into contact with the upper surface of the workpiece 100. In this case, the pressure of the pressure actuator 136 is uniformly applied to the movable electrode 135 and the fixed electrode 134 via the base 132 and the C-shaped yoke 133.
Subsequently, the base 132 is lifted upward by the servomotor 138. This upward lifting of the base 132 causes the pressure FL from the fixed electrode 134 to increase by an amount equivalent to how much the base 132 is lifted upward, whereby the pressure FU from the movable electrode 135 becomes smaller than the pressure FL from the fixed electrode 134 (FU<FL).
As a result, when electricity is applied between the movable electrode 135 and the fixed electrode 134, the current density at the joint section between the thin plate 101 and the first thick plate 102 increases, causing the calorific value to become relatively higher than the calorific value at the joint section between the first thick plate 102 and the second thick plate 103. Consequently, a good nugget extending from the thin plate 101 to the second thick plate 103 without an uneven amount of weld penetration is formed, thereby ensuring the weld strength.
In Japanese Unexamined Patent Application Publication No. 2003-251468 described above, the tip diameter of the movable electrode 121 that comes into contact with the thin plate 101 is made smaller than the tip diameter of the fixed electrode 122 that comes into contact with the second thick plate 103 so that the current density in the electric path between the movable electrode 121 and the fixed electrode 122 gradually decreases from the movable electrode 121 towards the fixed electrode 122, thereby increasing the weld strength between the thin plate 101 and the first thick plate 102.
However, the current density in the electric path between the movable electrode 121 and the fixed electrode 122 varies depending on the pressures from the movable electrode 121 and the fixed electrode 122, the thicknesses of the thin plate 101, the first thick plate 102, and the second thick plate 103, and the shape or the area of the workpiece 100. This makes it difficult to ensure uniform weld quality. Furthermore, using various movable electrodes 121 and fixed electrodes 122 having different tip diameters in an interchangeable manner in accordance with the thicknesses of the thin plate 101, the first thick plate 102, and the second thick plate 103 and the shape or the area of the workpiece 100 is not practical since it is extremely troublesome and can possibly lead to a significant decrease in productivity. In addition, the preparation and management of such various movable electrodes 121 and fixed electrodes 122 having different tip diameters require a large amount of management cost.
In Japanese Unexamined Patent Application Publication No. 2003-251469, on the other hand, the spot-welding device 130 is moved to each spot-welding position of the workpiece 100 supported by the clamper 128 so as to bring the fixed electrode 134 into contact with the second thick plate 103 of the workpiece 100 and to bring the movable electrode 135 into contact with the thin plate 101. Moreover, the pressure FU from the movable electrode 135 is set to be smaller than the pressure FL from the fixed electrode 134 by lifting the base 132 upward so that the current density between the thin plate 101 and the first thick plate 102 becomes relatively higher. Thus, a sufficient calorific value can be ensured at the joint section between the thin plate 101 and the first thick plate 102, thereby achieving an increased amount of weld penetration and increased weld strength.
However, in order to set the pressure FU from the movable electrode 135 to be smaller than the pressure FL from the fixed electrode 134 by moving the base 132 in a state where the workpiece 100 clamped by the clamper 128 is held and pressed between the fixed electrode 134 and the movable electrode 135, a large load is required on the clamper 128 that clamps the workpiece 100. On the other hand, when the clamped position of the workpiece 100 by the clamper 128 and the welding position, i.e., the spot-welding position, of the workpiece 100 are distant from each other, the workpiece 100 becomes bent. This causes the pressure FL from the fixed electrode 134 and the pressure FU from the movable electrode 135 to become unbalanced, making it difficult to ensure stable contact resistance between the thin plate 101 and the first thick plate 102 and stable contact resistance between the first thick plate 102 and the second thick plate 103. This can possibly result in variations in the current density at the joint sections of the workpiece 100, leading to reduced spot-welding quality. Furthermore, this method is limited to specific spot-welding devices since the method cannot be used in a spot-welding device that has an equalizing function between the base and the wrist portion of the robot for allowing for movement in response to a reaction force generated during the welding and pressing process.